diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -10968,6 +10968,8 @@
 def note_module_import_not_at_top_level : Note<"%0 begins here">;
 def err_module_self_import : Error<
   "import of module '%0' appears within same top-level module '%1'">;
+def err_module_self_import_cxx20 : Error<
+  "import of module '%0' appears within its own %select{interface|implementation}1">;
 def err_module_import_in_implementation : Error<
   "@import of module '%0' in implementation of '%1'; use #import">;
 def err_module_partition_import_global : Error<
@@ -11003,6 +11005,8 @@
 def err_export_not_in_module_interface : Error<
   "export declaration can only be used within a module interface unit"
   "%select{ after the module declaration|}0">;
+def err_export_partition_impl : Error<
+  "module partitions cannot be exported">;
 def err_export_in_private_module_fragment : Error<
   "export declaration cannot be used in a private module fragment">;
 def note_private_module_fragment : Note<
diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -420,6 +420,20 @@
 
   checkModuleImportContext(*this, Mod, ImportLoc, CurContext);
 
+  // C++20 
+  // A module implementation unit of a module M that is not a module partition
+  // shall not contain a module-import-declaration nominating M.
+  // (for an implementation, the module interface is imported implicitly,
+  //  but that's handled in the module decl code).
+
+  if (getLangOpts().CPlusPlusModules && !ModuleScopes.empty() &&
+      ModuleScopes.back().Module->isModulePurview() &&
+      ModuleScopes.back().Module == Mod) {
+    Diag(ImportLoc, diag::err_module_self_import_cxx20)
+         << Mod->getFullModuleName()
+         << !ModuleScopes.back().ModuleInterface;
+  }
+
   // FIXME: we should support importing a submodule within a different submodule
   // of the same top-level module. Until we do, make it an error rather than
   // silently ignoring the import.
@@ -471,7 +485,12 @@
   if (!ModuleScopes.empty())
     Context.addModuleInitializer(ModuleScopes.back().Module, Import);
 
-  if (!ModuleScopes.empty() && ModuleScopes.back().ModuleInterface) {
+  // A module (partition) implementation unit shall not be exported.
+  if (getLangOpts().CPlusPlusModules && Mod && ExportLoc.isValid() &&
+      Mod->Kind == Module::ModuleKind::ModulePartitionImplementation) {
+    Diag(ExportLoc, diag::err_export_partition_impl)
+    << SourceRange(ExportLoc, NamePath.back().second);
+  } else if (!ModuleScopes.empty() && ModuleScopes.back().ModuleInterface) {
     // Re-export the module if the imported module is exported.
     // Note that we don't need to add re-exported module to Imports field
     // since `Exports` implies the module is imported already.
@@ -483,7 +502,9 @@
     // [module.interface]p1:
     // An export-declaration shall inhabit a namespace scope and appear in the
     // purview of a module interface unit.
-    Diag(ExportLoc, diag::err_export_not_in_module_interface) << 0;
+    Diag(ExportLoc, diag::err_export_not_in_module_interface)
+         << (!ModuleScopes.empty() &&
+             !ModuleScopes.back().ImplicitGlobalModuleFragment);
   } else if (getLangOpts().isCompilingModule()) {
     Module *ThisModule = PP.getHeaderSearchInfo()
                            .lookupModule(getLangOpts().CurrentModule,